Ammonia Stripper

ABSTRACT

An ammonia stripper has an aerator and heat exchanger tubing in a tank. The aerator and coil are preferably attached to a frame to form a removable cassette. The cassette may also include a lid for the tank. The tank is preferably rectangular and optionally can be made with the dimensions of a standard shipping container. In a process, water flows through the tank while air bubbles are produced through the aerator. A heating medium such as water flows through the heat exchanger tubing. A gas containing ammonia is withdrawn from a headspace of the tank.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalApplication No. 62/076,784 filed on Nov. 7, 2014. U.S. ProvisionalApplication No. 62/076,784 is incorporated by reference.

FIELD

This patent relates to separating ammonia from water.

BACKGROUND

International Publication Number WO 2013/091094, Organics and NutrientRecovery from Anaerobic Digester Residues, describes a system in whichsludge from an anaerobic digester is treated to recover nutrients.Ammonia in a liquid fraction of the sludge is recovered and used toproduce a concentrated acidic ammonium salt solution. This solution maybe mixed with a solid fraction of the sludge to produce anitrogen-enriched solid. Various methods and devices for strippingammonia are described. In one example, an ammonia stripper has a set ofcylindrical reactors arranged for liquid to flow through them in series.Each reactor has a jacket around its outer walls. Hot water flowsthrough the jackets to heat the liquid. A diffuser at the bottom of thereactor introduces air bubbles into the liquid. Gas including ammoniacollects in the headspace of each reactor and is removed by connecting agas outlet to the suction side of a pump.

INTRODUCTION

This specification describes an apparatus and process that can be usedfor stripping ammonia from an aqueous solution or mixture.

An ammonia stripper described in this specification has an aerator and aheat exchanger tube inside of a tank. Optionally, a large tank can bebroken into cells by one or more internal dividers. In a process, waterflows through the tank while air bubbles are produced through theaerator. A heating medium such as water flows through the heat exchangertube. A gas containing ammonia is withdrawn from a headspace of thetank.

This specification also describes a cassette having a heat exchangertube and an aerator attached to a frame. In use, the cassette is fittedto a tank such that the aerator and heat exchanger tube are immersed inwater in the tank. Optionally, the cassette may also have a lid thatcovers at least part of the tank, and is supported by the tank, when thecassette is installed. The frame may be suspended from the lid. Inanother option, there may have one or more latches on the lid or thetank for connecting the lid to the tank. In another option, the cassettemay have one or more lift fittings to allow a crane, hoist or otheroverhead lifter to install or remove the cassette.

In one example, an ammonia stripper has a tank that is rectangular andhas the dimensions of a standardized shipping container, for example anISO container. The tank has internal dividers that create a plurality ofcells. Each cell has an aerator and heat exchanger tube. The aerator andheat exchanger tube in a cell are preferably parts of a cassette asdescribed above. The ammonia stripper can be transported by any vehicle,or crane etc., that is configured to accept a shipping container.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an isometric view of a cassette for an ammonia stripper.

FIG. 2 is an isometric view of a six-cell ammonia stripper including thecassette of FIG. 1.

FIG. 3 is a cross section of the ammonia stripper of FIG. 2.

FIG. 4 is a top view of the ammonia stripper of FIG. 2

DETAILED DESCRIPTION

An ammonia stripper may alternatively be called an ammonia strippingunit or an ammonia separation unit. An ammonia stripper as described inthis specification uses heat and subsurface aeration to remove ammoniafrom water. The water may be in the form of an aqueous solution ormixture. For example, the water may be the liquid fraction (for examplea filtrate or centrate) produced by dewatering the sludge from ananaerobic digester.

The water being stripped is contained in a vessel, for example a tank.The vessel is generally enclosed in that the vessel defines a headspacethat can be connected to an outlet port. However, the vessel ispreferably not entirely sealed since that is unnecessary and wouldcomplicate the construction of the vessel and liquid flow through thevessel. In use, the headspace can be connected to the suction side of apump to evacuate a gas containing ammonia. This puts the headspace undera partial vacuum, which inhibits ammonia emissions even if the vessel isnot entirely sealed.

At least one aerator in the tank provides subsurface aeration. Theaerator may alternatively be called a sparger or diffuser. In a largetank, there may be many aerators dispersed across the bottom of thetank. The one or more aerators may be medium bubble aerators. The one ormore aerators are connected to a blower, which provides the subsurfaceair. In cold climates the subsurface air can be heated prior to enteringthe stripper using a gas to liquid heat exchanger, for example with heatprovided by liquid effluent from the ammonia stripper. In anotheroption, the subsurface air can be heated in a gas to gas heat exchangerwith heat provided by exhaust from an engine or boiler burning biogasproduced by an anaerobic digester that provides the feed water to theammonia stripper.

The ammonia stripper operates at above ambient temperature, for example50 degrees C. or more, or about 70 degrees C. or more. The ammoniastripper is heated by recirculating a heating fluid, for example water,through a heat exchanger tube in the tank.

The heat exchanger tube is a tube bent, folded, coiled or otherwiseconfigured to provide a length greater than the perimeter of the tankinside the tank.

Optionally, when the ammonia stripper is used in combination with ananaerobic digester, the heating fluid can be heated by biogas or bywaste heat produced by a generator or boiler burning the biogas. Inanother option, liquid effluent from the ammonia stripper may be used asthe heating fluid. In this case, the effluent passes through a heatexchanger or other heater outside of the tank and then through the heatexchanger tube in the tank before leaving the ammonia stripper.

Waste heat in the effluent, whether used as the heating fluid or not,can be recovered in some cases by a heat pump or by heat exchange withanother solid or fluid. For example, an anaerobic digester operatingunder thermophilic or mesophilic conditions may require its feed streamor a recirculating side stream to be heated, or solids in the digestatemay need to be heated for drying or to drive additional ammonia gas fromthem. Some of the effluent from the ammonia stripper can be fed to theanaerobic digester, directly or mixed with a feed stream, as dilutionwater, which also carries heat to the digester. In another option, wasteheat in the liquid effluent from the ammonia stripper can be used toheat ambient air flowing to a subsurface aerator in the ammoniastripper.

The ammonia stripper preferably operates with multiple cells, forexample 3 to 6 cells. The cells may alternatively be called stages. Eachcell may be located in a separate tank. Alternatively, walls or bafflescan define cells within a tank. A tank with multiple cells is typicallyless expensive to construct than multiple separate tanks. Flow betweencells can be provided by perforations in the walls or baffles or byother means such as a weir. Flow between the stages is preferably drivenby gravity but could be pumped. Water is fed into a first cell and flowsthrough the cells generally in series to an outlet from the last cell.However, there can be some back mixing resulting in a flow pattern thatis intermediate between continuously stirred and plug flow reactorconditions. One or more aerators are placed in each cell.

In a gravity flow arrangement, the outlet from the last cell can be usedto control the level of the water in the ammonia stripper. This controlmay be passive, for example by providing an overflow weir at apredetermined elevation. Alternatively, the water level may becontrolled more actively, for example by using a weir with a movablegate or by using a swinging or telescoping tube with a variable heightoutlet.

The effluent from the last stage may flow into a buffer tank tode-aereate the effluent or to provide a flow buffering volume. Thebuffer tank may be configured as another cell within a tank containingthe ammonia stripping cells. De-aerating the effluent is typicallynecessary before it can be pumped. Optionally, mechanical de-aerationcan be used in place of or to supplement de-aeration in the buffer tank.

The ammonia stripper is preferably used to remove ammonia without addingchemicals to increase the pH of the water. The subsurface aeration firstdrives carbon dioxide out of the liquid. When used to treat the liquidfraction of sludge from an anaerobic digester, carbon dioxide isreleased from ammonium bicarbonate in the water. The ammoniumbicarbonate is created in the digestion process and is in equilibriumwith the carbon dioxide content of the biogas in the digester headspace,which may about 30 to 45% carbon dioxide. Stripping carbon dioxideraises the pH of the water to 9.2 or higher. When heated at elevated pH,the majority of the ammonia in the water becomes unionized ammonia gas,which is then driven out of solution and into the stripper headspace bythe subsurface diffused air. Heating the ammonia stripper also helpsprevent phosphate salts in the feed liquid from settling as the pHrises. Cooling the liquid effluent after it exits the ammonia stripperallows these salts to be precipitated in a controlled location such as astorage tank.

Optionally, there can be an air inlet to the headspace of the tank. Asheadspace gasses are removed, a crossflow of air is created through theheadspace of the tank, above the surface of the liquid. The crossflowreduces the concentration of carbon dioxide in the headspace. Thecrossflow also reduces the ammonia concentration in the headspace at theinterface between water and air. Diluting the headspace with cross flowair facilitates ammonia removal due to the higher concentration gradientof ammonia between the liquid and the air above it. Some dilution airmay be provided by the lack of a perfect seal around the headspace ofthe tank. Adding a further air inlet to the headspace is optional since,although the rate of ammonia removal is increased, the gas removed fromthe headspace is diluted with air and has a lower concentration ofammonia gas. Crossflow, if any, may be at 15 to 45% of the total (crossflow and subsurface air flow) gas flow through the ammonia stripper.

Gasses in the headspace are removed with a pump, for example by negativepressure created by an induced draft fan or other blower connected tothe headspace. Optionally, the outlet end of the blower may be connecteddirectly to a downstream ammonia acid scrubber.

The liquid effluent from the ammonia stripper has reduced total nitrogencontent. This effluent may be disposed of or re-used optionally afterfurther treatment. For example, an anaerobic digester in an agriculturalsetting that receives high nitrogen feed such as manure may require lownitrogen dilution water. When an anaerobic digester is used to treatprimary or activated sludge from a wastewater treatment plant, a liquidfraction of the digester sludge is typically returned to the wastewatertreatment plant. Removing ammonia from this liquid before it returns tothe wastewater treatment plant reduces the denitrification load on thewastewater treatment plant as well as recovering nitrogen in a usableform.

Ammonia rich air is withdrawn from the headspace of the ammoniastripper. To recover the ammonia (and total nitrogen) in a useful form,the ammonia rich air can be sent to an ammonia scrubber. In one example,an ammonia acid scrubber uses a counter flow column configuration withammonia rich air circulating from the bottom up through a packed bed.The packed bed may have a plastic media to enhance gas/liquid masstransfer surface area. A liquid shower flows from the top down andreacts with the ammonia gas in the air stream. For example, a sulfuricacid shower can be used to form ammonium sulfate. Ammonium sulfate isstored in a sump at the bottom of the scrubber column. Ammonium sulfateis pumped from the sump for recirculation to the column. Sulfuric acidis added, for example to the sump or in the recirculating stream.Sulfuric acid addition can be controlled automatically based on a pH setpoint. Excess sulfuric acid can be added to the recirculation stream orto a product ammonium sulfate stream to produce an acidic ammoniumsulfate solution, which reduces ammonia volatilization if the ammoniumsulfate will be dried before use. The acid scrubber may produce about30% ammonium sulfate solution when no excess sulfuric acid is dosed.Optionally, this solution may be concentrated before it is used.Alternatively, water without acid can be recirculated through the columnto form ammonium hydroxide for use in place of ammonium sulfate.Ammonium sulfate or ammonium hydroxide can be used, for example, inliquid form as a fertilizer applied to land, as a liquid added tocompost to increase its nitrogen content, or in a dry form as part of asolid fertilizer.

The ammonia stripper may have a cassette based, or cassette and tankbased, construction. The cassette has a frame with a heat exchanger tubeand aerator attached to the frame. There may also be a lid for a tankattached to the frame. For example, the frame may be suspended from thelid. The lid may have one or more latches for attaching the lid to thetank. There may also be one or more lift fittings attached to the lid,or to the frame. The lift fittings allow the cassette to be lifted by,for example, a crane, boom truck, or hoist.

The cassette is used with a tank large enough to contain the aerator, atleast part of the heat exchanger tube, and at least part of the frame.Preferably, the cassette is designed to be compatible with a particulartank. For example, the lid can be sized to cover at least part of thetank. Alternatively or additionally, the lid can be designed so that thelid is supported by the tank. For example, the lid can rest on the wallsof the tank or on fittings attached to the tank. Assembling the ammoniastripper involves lowering the frame of the cassette into the tank untilthe lid bears on the tank. Optionally, latches on the tank or lid can beused to secure the lid to the tank.

An ammonia stripper may have a tank that is rectangular and has thedimensions of a standardized shipping container. One or more internaldividers create multiple cells in the tank. An aerator and a heatexchanger tube are placed in the tank, preferably in each cell. Theaerators and heat exchangers can be provided by adding a cassette asdescribed above to each cell.

Whether the tank is in the shape of a shipping container or not, a tankcan contain multiple cassettes with internal walls between thecassettes. The walls can be placed between each pair of cassettes or inother arrangements. The walls help to provide more nearly plug flowconditions through the tank. A wall can also be used to provide ade-aeration cell or buffer tank after the last cassette.

The ammonia stripper can be used to remove ammonia from a liquidfraction of anaerobic digester sludge. The ammonia can be used toproduce an ammonium salt solution. For example, the ammonia stripper canbe used in a nutrient recovery system as described in InternationalPublication Number WO 2013/091094, which is incorporated by reference.

In one example, the design of a very large ammonia stripper required 15jacketed reactors as described in International Publication Number WO2013/091094.

Replacing the jacketed reactors with a cassette-based design as shown inFIGS. 1 to 3 reduced the expected capital cost by 40%. The reduction incost results partially from a reduction in materials and partiallybecause it is easier to assemble the heat exchanger tubes and attachthem to a frame than to make a heating jacket around a tank. Thecassette-based design is also expected to be easier to maintain than ajacketed reactor since the cassettes can be removed for maintenance.

FIG. 1 shows an example of a cassette 10. The cassette 10 has a frame 12suspended from a lid 30. The lid 30 has a set of lift fittings 14 thatcan be attached to a chain or cable from a crane, hoist or other similardevice. The cassette 10 also has a heat exchanger tube 18 attached tothe frame 12. The heat exchanger tube 18 has a number of bends, forexample to produce the coiled shape shown. The ends of the heatexchanger tube 18 are connected to or provide an inlet 20 and an outlet22. The cassette 10 also has at least one aerator 24. The aerator 24 isattached to the frame 12, preferably near the bottom of the frame 12. Inthe example shown in FIG. 1, several aerators 24 are attached to theframe 12 though a manifold 26. The manifold 26 is in turn connected toan air feed pipe 28.

FIGS. 2 to 4 show a tank 40 divided into a set of cells 42. The tank isconstructed of walls 46 connected to a tank frame 44 extending upwardsfrom a floor 45. The tank frame 40 is preferably sized and fitted withfittings 48 according to the standards for a standardized shippingcontainer. Each cell 42 has a cassette 10. The lid 30 of each cassette10 has a set of latches 16 that can be used to secure the lid 30 to acell 42. The lid 30 is latched to the walls of the tank 40 and providesa cover for the cell 42. A tank inlet 50 communicates with a cell 42 onone end of the tank 40. An optional buffer tank 46 is provided at theother end of the tank 46. The buffer tank 46 is provided with a tankoutlet 52. The buffer tank 46 and the last cell 42 also have transferports 51 which may be fluidly connected together.

FIG. 3 shows the interior of the tank 40. Interior walls 54 connected tothe frame 44 separate the cells 42 from each other. The interior walls54 are perforated with one or more openings. In one example, there is anopening at the base of the wall and also additional openings higher inthe wall. The liquid being stripped flows into the first cell 42 throughthe tank inlet 50. The liquid then flows between cells 42 through theopenings in the interior walls 54. An opening near the base of theinterior walls 54 can also be used to allow liquid to flow between thecells 42 when the tank 40 is filled or drained for maintenance.

A buffer tank wall 56 separates the last cell 42 from the buffer tank46. The buffer tank wall 56 does not have openings. A telescoping valve(not shown) connected between the transfer ports 51 provides flowbetween the last cell 42 and the buffer tank 46 while controlling theliquid level in the system cells 42. Treated liquid collects in thebuffer tank 46 and may at least partially de-aerate before being drainedout through the tank drain 52. The buffer tank 46 is not heated and doesnot have an aerator.

Ammonia rich gas is withdrawn through gas outlet 60. Optionally theremay be more than one gas outlet 60, for example one or more outlets ineach cell 42 or multiple outlets all in one cell 42 or a subset of thecells 42. Interior walls 54 do not extend to the lids 30 and so ammoniarich gas can be withdrawn from all of the cells 42 through one or moreoutlets 60 in one cell 42. In the example shown, there is no inlet forsweeping air, but there may be some mild leakage around seals betweenthe lids 30 and the tank 40. The ammonia gas flows through the tank 40to gas outlet 60 in a direction opposite to the flow of liquid throughthe tank 40. This helps ensure that ammonia rich gas does not mix withthe low ammonia concentration liquid in the last cell 42.

As discussed above, an ammonia stripper has a tank, with an aerator anda heat exchanger tube inside of the tank. The tank also defines aheadspace. An outlet in communication with the headspace allows ammoniarich gas to be removed from the headspace. The tank may be rectangular.Whether rectangular or not, the tank may be divided into stages,alternatively called cells, by one or more internal dividers. To stripammonia from water, the water is added to the tank. Air bubbles areproduced in the water.

A heating medium flows though the tube. Gas containing ammonia isproduced in the headspace and withdrawn from the headspace.

1. An ammonia stripper comprising, a) a tank; b) an aerator inside ofthe tank; and, c) heat exchanger tubing inside of the tank.
 2. Theammonia stripper of claim 1 wherein the tank defines a headspace andfurther comprising an outlet in communication with the headspace.
 3. Theammonia stripper of claim 2 wherein the tank is rectangular.
 4. Theammonia stripper of claim 1 wherein the tank is divided into cells byone or more internal dividers.
 5. A process for stripping ammonia fromwater comprising steps of, a) providing a tank having heat exchangertubing below a headspace; b) adding water to the tank such that the heatexchanger tube is immersed; c) producing air bubbles in the water; d)flowing a heating medium through the heat exchanger tube; e) withdrawinga gas containing ammonia from the headspace of the tank.
 6. A cassettecomprising, a) a frame; b) a heat exchanger tube attached to the frame;and, c) an aerator attached to a frame.
 7. The cassette of claim 6further comprising a lid attached to the frame.
 8. The cassette of claim7 wherein the frame is suspended from the lid.
 9. The cassette of claim6 further comprising one or more latches attached to the lid.
 10. Thecassette of claim 6 further comprising one or more lift fittingsattached to the lid or to the frame.
 11. The cassette of claim 6 incombination with a tank sized to contain at least part of the frame. 12.The cassette of claim 11 wherein the lid is sized to cover at least partof the tank.
 13. The cassette of claim 11 wherein the lid is supportedby the tank.
 14. An ammonia stripper comprising, a) a tank that isrectangular and has the dimensions of a standardized shipping container;b) an aerator in the tank; c) heat exchanger tubing in the tank.
 15. Theammonia stripper of claim 14 having internal dividers that create aplurality of cells within the tank.
 16. The ammonia stripper of claim 15wherein each cell has an aerator heat exchanger tubing.
 17. The ammoniastripper of claim 14 wherein the aerator and heat exchanger tubing areattached to a frame of a cassette.